1. Field of the Invention
This invention relates to conductive resin composition excellent in wear resistance and conductivity and an encoder switch that uses this conductive resin composition.
2. Description of the Related Art
Conventional conductive resin composition used for encoder switches that is prepared by a method in which heat resisting resin such as phenol resin, phenol-aralkyl resin, or polyimide resin is dissolved in an organic solvent to prepare a resin solution, conductive filler such as graphite, carbon black, or carbon fiber is mixed and dispersed in the resin solution to prepare a conductive ink, and the conductive ink is coated on a substrate consisting of material such as Bakelite or alumina by means of screen printing has been used. Conductive carbon black such as acetylene black, Ketjen black or furnace black is generally used as the conductive filler. If the product is required to be rendered more conductive, graphite is added, and if the product is required to be rendered more wear resistant, carbon fiber is added.
However, as the electronic parts have been required to be miniaturized and rendered long life recently, the material described hereinabove has been insufficient for these requirements. In detail, in the case of the material containing graphite, the product is easily shaved because of cleavage characteristic of graphite, and the service life of an encoder switch that employs a conductive resin composition containing graphite is as short as 10,0000 cycles or less because of poor wear resistance of the material. The conductive resin composition containing carbon fiber cannot be employed in applications that involve a fine pattern due to miniaturization of electronic parts because of dimensional problems including a diameter of several xcexcm and length of several ten xcexcm (though the sliding characteristic is good because of the hardness of the carbon fiber.) A trial in which graphite fibril or carbon nanotube, namely fine carbon fiber, is used has been made, but the sliding resistance is not improved because the entanglement of fine carbon fiber cannot be disentangled in a resin solution. Furthermore, expensive cost of fine carbon fiber is also the barrier for application.
It is the object of the present invention to provide conductive resin composition excellent in wear resistance and conductivity and accommodable for fine pattern.
Conductive resin composition of the present invention contains conductive filler such as carbon beads and carbon black in a binder resin consisting of phenol resin, and the content of the conductive filler ranges from 34 to 60% by weight.
The binder resin allows carbon beads or carbon black to disperse homogeneously therein and cure-shrinks to thereby increase the contact pressure between conductive filler particles, and to reduce the contact resistance between particles, and as the result the conductivity is improved. To obtain the effect described hereinabove, phenolic resin is preferably used as the binder resin. Phenol resin gives sufficient cure shrinkage, the cure shrinkage renders the contact pressure between conductive filler high and the contact resistance between particles low, and thus the conductivity of the conductive resin compound is improved.
Examples of the phenolic resin include, for example, resol-type phenol resin, novolak-type phenol resin, phenol aralkyl resin, xylene-modified phenol resin, cresol-modified phenol resin, furan-modified phenol resin, epoxy-phenol resin, and phenol-melamine resin.
Carbon beads render the conductive resin composition conductive and play a role as the structural material for improving the wear resistance of a slide brush consisting of the conductive resin composition that bears the weight of the slide brush.
If carbon beads only are used as the conductive filler of the conductive resin composition, the conductivity of the conductive resin composition is low because of not many contact points between particles due to spherical shape of the carbon beads and high resistance of the carbon beads itself.
Use of carbon beads and carbon black together as the conductive filler of the conductive resin composition allows carbon black to interpose between carbon beads to thereby increase the contact point between conductive fillers, and thus plays a role to improve the conductivity of the conductive resin composition.
When conductive resin composition containing carbon beads as the structural material is used for an encoder switch, projections of carbon beads and recesses of binder resin are formed on the conductive resin composition surface namely the sliding surface of the slide brush. When a slide brush rides on a projection, noise is generated in the output signal of the encoder switch. Furthermore, because carbon beads are too hard, the carbon brush wears due to sliding-contact with carbon beads to result in shortened service life of the encoder switch.
Use of carbon beads and carbon black together as the conductive filler of the conductive resin composition renders the recess conductive because carbon black interpose between carbon beads. Furthermore, because carbon black adheres on the carbon bead surface, the direct contact between the slide brush and carbon beads is avoided.
The content of the conductive filler, containing carbon black and carbon beads, of the conductive resin composition lower than 35% by weight results in the low conductivity and high specific resistance of the conductive resin composition due to insufficient quantity of the conductive filler.
If such conductive resin composition having high specific resistance is used for an encoder switch, a fixed resistor having high resistance must be used as the pull-up resistance used for obtaining the pulse waveform. Therefore, a current flows little when turned on, and the effect of the external noise becomes effective.
On the other hand, the content of the conductive filler, containing carbon black and carbon beads, of the conductive resin composition exceeding 60% by weight result in the insufficient increase of contact pressure between conductive filler particles due to cure shrinkage of binder resin and results in the high contact resistance between particles because of insufficient quantity of the binder resin, and thus results in the low conductivity. Furthermore, the insufficient quantity of the binder resin renders the conductive resin composition brittle. If such conductive resin composition is used for an encoder switch, the slide brush slides on the conductive resin composition to result in the breakdown of the conductive resin composition and results in shortened service life of the encoder switch.
The conductive filler content in the conductive resin composition of the present invention preferably ranges from 45 to 51% by weight. The conductive filler content in the range as described hereinabove brings about the low specific resistance and also long service life of an encoder switch for which the conductive resin composition is used.
In the conductive resin composition of the present invention, the ratio of the carbon beads to the carbon black is preferably 1 to 8 by weight. The ratio of carbon beads to carbon black as described hereinabove renders the conductive resin composition both conductive and wear resistant, and both the less noise output signal and the long service life of the encoder switch for which such conductive resin composition is used are realized.
In the conductive resin composition of the present invention, the carbon beads are spherical, and particle diameter of the carbon beads ranges preferably from 1 to 30 xcexcm. Because the carbon beads are spherical, the sliding surface of a slide brush is covered with an aggregation of spherical surface, and the slide brush wears little.
The carbon beads having the particle diameter smaller than 1 xcexcm cannot bear the weight of the slide brush, the wear resistance of the conductive resin composition is poor, on the other hand the carbon beads having the particle diameter larger than 30 xcexcm results in the projection of the carbon beads from the border of the conductive pattern formed of the conductive resin composition, and results in the poor pattern size accuracy.
In the present invention, the conductive resin composition contains the carbon beads that are formed by heating and carbonizing thermosetting resin powder, at that time the heat treatment temperature is in a range from 700 to 1200xc2x0 C. Such carbon beads are accurately spherical and conductive.
For example, phenol resin, benzo-guamine or the like may be used as the thermosetting resin. The heat treatment temperature for treating the thermosetting resin lower than 700xc2x0 C. results in the insufficient carbonization of the thermosetting resin and the high specific resistance of the resultant carbon beads, and thus results in the poor conductivity of the conductive resin composition. On the other hand, the heat treatment temperature higher than 1200xc2x0 C. results in the cleavage of the carbon beads due to strained rearrangement of molecules during carbonization like cracked pomegranate. Edges of the cleaved carbon beads projected on the surface of the conductive resin composition cause wear of a slide brush.
The conductive resin composition of the present invention contains carbon beads and carbon black in the carbon binder resin comprising xylenic resin and phenolic resin, and the ratio of the phenolic resin to the xylenic resin ranges preferably from 1 to 33 by weight.
Xylenic resin is served as not only as the binder resin but also as a functional resin for rendering the theological characteristic of the conductive ink that contains the conductive resin composition and organic solvent suitable for fine pattern forming by means of screen printing.
Generally in the dispersion that contains particles, the larger the particle diameter is and the more spherical the particles are, the higher the flow property is. Because generally the carbon bead is perfectly spherical in shape and has a particle diameter equal to or larger than 1 xcexcm, the fluidity of the conductive ink comprising the conductive resin composition containing the carbon beads and an organic solvent is high. Too high fluidity of the conductive ink results in the sagging and bleeding of the ink at the edge of the pattern when the ink is screen-printed to form a conductive pattern, and such conductive ink is not suitable for forming a fine pattern.
The ink containing xylenic resin behaves as described hereunder in screen printing process. The viscosity is low while the ink is being subjected to a load loaded by means of a squeegee against a screen so that the ink is suitable for printing, on the other hand the viscosity becomes high so that the ink does not sag after the ink is extruded through the screen mesh and the load is unloaded. As the result, because the conductive ink maintains the pattern shape continuously as screen printed, it is possible to form a fine pattern.
The ratio of phenolic resin to xylenic resin smaller than 1 by weight, that is, in the case where the proportion of phenolic resin is larger than the proportion of xylenic resin, results in the separation of xylenic resin from the phenolic resin and the bleeding of the xylenic resin on the conductive resin composition. On the other hand, the ratio of phenolic resin to xylenic resin larger than 33 by weight, that is, in the case where the proportion of phenolic resin is large and the proportion of xylenic resin is small, results in the delayed restoration time from extrusion of the conductive ink through the screen mesh to restoration of high viscosity, and could result in the sagging and bleeding of the pattern edge.
Because the conductive resin composition of the present invention as described hereinabove is used for an encoder switch of the present invention, in the case of the encoder switch of the present invention, the signal noise is little and the service life is long, and the small-sized encoder switch is realized because the fine conductive pattern can be formed.